I work for John Deere, though I don't speak for the company. Every time I tell a farmer I work with the displays they tell me that they won't plant without the GPS. They will sometimes drive the combine by hand, but there is enough additional profit in GPS planting that nobody will drive a planter by hand. Corn does is very picky about how close it is planted to other plants - too close or too far apart will reduce yields and the GPS can optimize that by enough to be worth it.
Even in the combine case, if there is bad storm damage they won't drive by hand as the GPS can find the rows and thus harvest a lot more crop. Every couple years there will be a field storm damaged bad enough that it would be a total loss if they had to drive the combine by hand, but the GPS can find the rows and so there isn't even an insurance claim (crop insurance insures you will get your average yields over the previous 10 year, so if you have a total loss crop that brings your average yields way down for the next 10 years which means a future disaster crop gets bad payout)
Even in the combine case, if there is bad storm damage they won't drive by hand as the GPS can find the rows and thus harvest a lot more crop. Every couple years there will be a field storm damaged bad enough that it would be a total loss if they had to drive the combine by hand, but the GPS can find the rows and so there isn't even an insurance claim (crop insurance insures you will get your average yields over the previous 10 year, so if you have a total loss crop that brings your average yields way down for the next 10 years which means a future disaster crop gets bad payout)